The invention is based on a priority application EP 02360129.7 which is hereby incorporated by reference.
The invention deals with a method for remodulation of a modulated optical signal using a disturbed line signal and an optical clock signal derived from the undisturbed original line signal modulated with the bitrate frequency, feeding both signals in a Raman amplifying fiber connected to at least one Raman pump and running the clock signal as Raman pump wavelength for the line signal.
The invention is also related to a device for remodulation of optical signals with an input for line signals and a coupler, the coupler connects the line and a Raman pump laser with a piece of Raman active fiber, and the Ramon pump laser is connected to a clock recovery device which modulates the Raman pump laser light with the bit rate frequency of the optical signal.
The invention is also related to a transmission system for transmission of optical signals in transmission lines comprising transmitter means, receiver means and regeneration means in the line where the regeneration means have devices to recover amplitude, clock and shape of the signals, the clock recovery device comprises means to recover an optical clock signal, the said optical clock signals is connected to a remodulation device as described in the invention.
Optical communication systems are a fast-growing constituent of communication networks. The term “optical communication system” as used in the following relates to any system or device which makes use of optical signals to transport information across an optical waveguiding medium. Optical communication systems comprise inter alia telecommunication systems, local area networks (LAN), cable television systems etc. Optical communications at ultra-high bit-rates with bit rates for example over 40 Gbit/s over long distance suffers from severe degradations occurring during propagation.
For the transmission capacity of optical fibres in optical communication systems is expected to advance in the future, the evolution of optical signal recovery is one of the core technologies involved in this process. A key to this evolution is the availability of extremely-broad-band optical devices, offering modulation, regeneration, amplification over nearly all the transmission window allowed by silica. In this respect, optical regeneration by synchronous modulation was proven to alleviate limitations of transmission due to reduction of timing litter, decreasing polarisation mode dispersion impact, improvement of optical signal-to noise ratio at low bit error rate levels.
For regeneration of optical signals in transmission system several solution are known.
Electro-optical interferometers are well known. But this InP or LiNbO3 MZ devices, suffer from rather high insertion loss (>10 dB for InP, >8 dB for PI-concatenated LiNbO3), which in turn limits or even suppresses the benefit of synchronous optical regeneration. They also face a problem from the operating speed limit imposed by electronic circuitries and electrode phase-matching which is typical 40 GHz. Their power consumption is rather high (>10 W), which limits their interest for submarine-cable applications. Finally, their optical power-handling capability is limited (<100 mW), which prevents massive/simultaneous WDM optical regeneration.
Also all optical semiconductor amplifiers are known.
For example the EP 0 975 106 discloses a system where a digital modulation is extracted for regeneration using an optical coupler and clock extractor, and semiconductor amplifiers are used in an amplitude or phase modulator arrangement to modulate soliton pulses. The device comprises an optical coupler and electro-optical recovery unit for recovery of the clock signal from the incoming signal in the input line (F1). This permits the extraction of digital information for regenerating use and a means of modulation of the amplitude and/or the phase of the solitons by the clock signal. The device is characterized by the use of a Mach-Zehnder modulator containing first and second optical semiconductor amplifiers arranged in two parallel arms with the gain of each modulated by the recovered clock signal.
The output signals from the recovery unit are fed to a splitting circuit providing, through delay circuits feedback to amplifiers in the modulator. The output line contains the-signal of same wavelength as the input line, with the clock signals blocked by the feedback signals.
Solution with semiconductor optical amplifiers as described in EP 0 975 106 face the problem that the all-optical components can operate beyond 40 GHz but at the expense of complexity.
Basically all the aforementioned approaches require costly semiconductor of electro-optic components, which are difficult to manufacture, to package and to optimize.